In general, inkjet printing machines or printers include at least one printhead that ejects drops or jets of liquid ink onto a recording or image forming surface. An aqueous inkjet printer employs water-based or solvent-based inks in which pigments or other colorants are suspended or in solution. Once the aqueous ink is ejected onto an image receiving surface by a printhead, the water or solvent is evaporated to stabilize the ink image on the image receiving surface. When aqueous ink is ejected directly onto media, the aqueous ink tends to soak into the media when it is porous, such as paper, and change the physical properties of the media. Because the spread of the ink droplets striking the media is a function of the media surface properties and porosity, the print quality will be inconsistent. To address this issue, indirect printers have been developed that eject ink onto a blanket mounted to a drum or endless belt. The ink is dried on the blanket and then transferred to media. Such a printer avoids the changes in image quality, drop spread, and media properties that occur in response to media contact with the water or solvents in aqueous ink. Indirect printers also reduce the effect of variations in other media properties that arise from the use of widely disparate types of paper and films used to hold the final ink images.
In aqueous ink indirect printing, an aqueous ink is jetted onto the blanket on the endless drum or belt. Applying a coating material to the blanket can facilitate the wetting of the blanket surface with ink drops and the release of the ink image from the blanket surface. Coating materials have a variety of purposes that include wetting the blanket surface, inducing solids to precipitate out of the liquid ink, providing a solid matrix for the colorant in the ink, and aiding in the release of the printed image from the blanket surface. After the ink image has been formed on the coating on the blanket, the coating and ink are at least partially dried to help put the image and coating on the blanket in condition for transfer to media in the transfer nip.
One challenge that arises in this printing process is the adherence of the dried coating and ink to the blanket of the belt or drum. This adherence tends to hold media sheets firmly to the blanket after the sheet exits the transfer nip. Previously known indirect aqueous inkjet printers that print media sheets use technology from the offset printing industry known as mechanical gripper bars to pry the sheets from the blanket. That is, the gripper bars physically clamp the leading and trailing edges of media sheets after they pass through the transfer nip to provide a positive “stripping” force that peels the media away from the blanket. These gripper bars are fixed pitch devices, which means that the gripper bars are designed for a maximum sheet length in the process-direction. Thus, when the printer is used to print sheets smaller in length than the length for which the gripper bars were designed, sheets remain stuck to the blanket and productivity loss occurs. This productivity loss is proportional to the difference between the maximum sheet length and the sheet length being processed at the time. Additionally, gripper bar assemblies are expensive and large, which adversely impacts the price and size of the aqueous printer in which they are used. Less expensive and flexible mechanisms for stripping media from blankets in aqueous inkjet printers that print media sheets would be beneficial.